Crane overload safety device with temperature compensation

ABSTRACT

A crane overload safety device including at least one pivoted arm connected to a strain measuring device adapted to be fixed to a loaded part of the crane. The pivoted arm is provided with one or more photocell devices or the like and cooperates with at least one cam disc. The strain measuring device is in the form of a tension rod dynamometer both ends of which are adapted to be attached to the loaded part of the crane by multipoint suspension devices.

United States Patent 1191 Eiler Sept. 10, 1974 CRANE OVERLOAD SAFETYDEVICE WITH TEMPERATURE COMPENSATION Primary Examiner-Even CLBlunkAssistant ExaminerJames Rowland [75] Inventor 22223:? DuSSeldOrf-BemathAttorney, Agent, or Firm-Toren and McGeady [73] Assignees Leo GottwaldK.G.,

Dusseldorf-Holthausen, Germany [22] [57] ABSTRACT [21] A crane overloadsafety device including at least one pivoted arm connected to a strainmeasuring device [30] Foreign Application Priority Data adapted to befixed to a loaded part of the crane. The Apr. 21, 1971 Germany 2119279pivoted arm is Provided with one or more photocell devices or the likeand cooperates with at least one 52 us. Cl 212/39 MS, 340/267 0 camdisc- The Strain measuring device is in the form of [51 Int. Cl. B66c13/48 a tension rod dyhamometer both ends of Which are 5 Field of Search33/147 D, 148 D 5 adapted to be attached to the loaded part of the crane73 R 1 2 2 9; 2 7 C by multlpoint suspension devlces.

[561 References Cited 9 Claims, 5 Drawing Figures UNITED STATES PATENTS3,754,665 8/1973 Eiler 212/39 MS r: 1 l "v' I 131.

PATENTEBSEP 1 own SHEEI 1 BF 4 PAIENIEB SEP 1 0:914

, 3.934.550 SHEU 3 OF 4 CRANE OVERLOAD SAFETY DEVICE WITH TEMPERATURECOMPENSATION This invention relates to overload safety devices forcranes, particularly for mobile cranes mounted on motor driven vehicles.

Cranes, particularly those mounted on motor driven vehicles, need to beequipped with overload safety devices for preventing accidents. This isnecessary not only from practical experience but also to comply withrelevant legal regulations. Several forms of overload safety deviceshave already been proposed. Existing overload safety devices areintended in particular to ensure that the crane, mounted on its motordriven vehicle, cannot topple over in operation. This ensurance is ofparticular importance in the operation of cranes mounted on turntable.In cranes of this kind the highest permissible crane load is limited bythe stability of the crane, particularly when the jib extends sidewaysfrom the longitudinal axis of the vehicle. To ensure safety the loadlifted by the crane must never be as high as 100 percent of the ultimateload determined by the stability of the vehicle on the ground.

All the safety devices which have hitherto been proposed for givingwarning signals and for actuating systems for preventing furtheroperation of the crane, when the permissible load limit is approached,involve the use of electronic devices, in particular transmitters whichdeliver signals representing stresses, moments and geometric quantities,for example jib angles, jib extension lengths and the like. Thetransmitters operate on known comparison principles in which themeasured value of a current or voltage is compared with a specifiedvalue. The measured values are processed by a central computing devicewhich continuously compares them with stored specified values. As soonas the permissible load is reached signals are automatically deliveredwhich prevent operation of all the crane functions which could increasethe load still further. The advantage of this is that the crane operatordoes not need to observe signals representing stresses and geometricquantities, during operation of the crane and compare the indicatedvalues with the permissible values, which has to read from tables, sothat he can adapt the further operation of the crane to the prevailingconditions and safety regulations.

However, although existing overload safety devices do relieve theoperator from the need to read signals and consult tables, neverthelessthey have considerable disadvantages in that they are not well adaptedto the conditions which prevail in the operation of cranes mounted onmotor driven vehicles. In the first place,

' the highly sensitive electronic transmitters and computing devices aretoo fragile to withstand the rough working conditions of a crane, orthey have an insufficiently long working life. Furthermore, devices ofthis kind are costly and when they develop faults repairs can beeffected, if at all, only at considerable cost in time and money.Repairs usually involve a dismantling of the entire device byspecialists who have to be brought in for the purpose.

I have previously proposed an overload safety device for a crane, thedevice being less costly to manufacture, of more robust and reliableconstruction, and having greater ease of installation on a crane withoutrepairing specialized technical knowledge. Such prior device included atleast one pivoted arm connected to a strain measuring device which isadapted to be fixed to a loaded part of the crane, the pivoted arm beingprovided with one or more photocell devices or the like and cooperatingwith at least one cam disc.

Thus this prior device avoids the use of delicate electronic devices andenables an overload safety device to be constructed using mechanicalparts shaped to represent operational parameters, the mechanical partsthemselves in the form of the cam disc or discs performing evaluationand comparision functions in such a way that when the permissible loadlimits are reached signals are delivered which prevent the operation offurther load-increasing crane functions.

During operation of the crane the position of the pivoted arm isdetermined at any instant by the strain measuring device in dependenceon the forces acting on the crane at that instant. The pivoted arm oreach pivoted arm cooperates, by means of its photocell device or thelike, with the edge of thecam disc, which has a shape representing thepermissible values of the relevant parameters, for example the jibangle, the length of the jib extension, the direction of the jib planerelative to the vehicle in the case of a mobile crane, and the nature ofthe support of the vehicle on the ground. The edge of the cam discinterrupts a beam such as a light beam of the photocell device or thelike when the permissible load is reached.

However, although this prior overload safety device operates entirelysatisfactorily, nevertheless it has been found that the indicationsgiven by the device and the controlling action of the device aresubjected, under certain circumstances, to temperature influences whichcan have disadvantageous effects. For example, if the dynamometer is inthe shade while the stressed constructional part of the crane is beingwarmed by the sun, different thermal expansions occur which aresuperposed on the changes in length produced by the crane loads.Consequently, the overload safety device responds either too early ortoo late.

The object of the present invention is to improve the overload safetydevice so as to allow it to function independently of temperaturechanges. The invention starts out with the idea of utilizing the stresseffects in the loaded structural part of the crane for the temperaturecompensation by suitably attaching the dynamometer to the loadedstructural part, using a compensating attachment. The problem is solvedaccording to the invention in that both ends of the strain measuringdevice, which is in the form of a tension rod dynamometer, are arrangedto be attached to the loaded structural part of the crane by multipointsuspension devices. What is essential in the invention is therefore thatthe tension rod dynamometer is attached to the loaded structural part atgeometrically separate points.

Each multipoint suspension device preferably consists of two linkspivoted to the end of the tension rod dynamometer preferably on a commonaxis, the other ends of the links being arranged to be attached to theloaded structural part. It has been found thatby this method ofattachment the different thermal expansions between the dynamometer andthe loaded part are compensated as though the two structures are both atthe same temperature.

A suspension device which is particularly simple to install on the craneis provided if the attachment points of the links on the loaded part ofthe crane are situated on either side of the longitudinal axis of thedynamome ter, symmetrically with respect to this axis.

The method of attachment of the overload safety device has the furtheradvantage that a particularly simple means of adjustment can be arrangedfor adjusting the movement ratio between the movement of the pivoted armand the movement of the pivoted lever of the dynamometer. The pivotedarm is connected to the pivoted lever of the dynamometer by a cable. Itis necessary to adjust the movement ratio because the thicknesses of thestructural parts of cranes vary from crane to crane, within themanufacturing tolerances. Consequently, for a given crane load, andassuming given crane load conditions, the change in length of thedynamometer tension rod varies from crane to crane. This can falsify themeasured values and the switching off action of the overload safetydevice. Before putting a crane equipped with an overload safety deviceinto operation it is therefore necessary to calibrate the overloadsafety device, in order to ensure that under given load conditions thepivoted arm takes up the correct position relative to the disc cam.Hitherto the calibration has been effected by selecting a roller orsegment disc of suitable diameter. This roller or segment disc ismounted on the axle of the pivoted arm and supports the cable connectingthe pivoted arm to the pivoted lever on the dynamometer. The suspensiondevice according to the invention allows the calibration to be effectedwithout any necessity to exchange a part of the overload safety device,or at least allows a coarse calibration to be ob tained. For thispurpose at least one end of the dynamometer may have a slot in which thepivot pin for the ends of the links connected to the dynamometer isadjustable in position by means of an adjustment screw. The two linksare preferably made adjustable in length.

An example of an overload safety device in accordance with the inventionwill now be described with reference to the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic side view of part of a crane jib incorporatingan overload safety device;

FIG. 2 is a section taken along the line Il-II in FIG.

FIG. 3 is a section taken along the line III--III in FIG.

FIG. 4 is a diagrammatic plan view of the tension rod dynamometeraccording to the invention attached to a crane jib; and,

FIG. 5 is a side view corresponding to FIG. 4.

As shown in the drawings an overload safety device 11 is attached bymeans of a bracket 13 to a crane jib 12, of which the drawings show onlya part. The overload safety device 11 has a housing 14 which issubdivided into two compartments 18 and 19 by a wall 17 which supportstwo shafts 15 and 16. The shaft 16 rotates in a bearing fixed to thewall 17 and extends into both compartments. The part of the shaft 16 inthe compartment 18 supports a number of cam discs 21. The rim of eachcam disc 21 has a complex'curved shape representing permissible jibloads for varying jib luffing angles a. Each cam disc 21 is divided intofour quadrants, each of which represents a particular crane operatingcondition, that is to say with the crane vehicle supported, for example,by outriggers; the crane vehicle unsupported; the jib in the plane ofvehicles longitudinal axis; and the jib in a plane perpendicular tovehicles longitudinal axis. Furthermore, each individual cam disc 21represents a particular amount of jib extension, for example when thejib is telescoped.

The part of the shaft 16 in the compartment 19 supports a pendulum plate22 which is free to rotate on the shaft 16. To the lower part of thependulum plate 22 a pendulum 24 is attached by an arm to which aphotocell or a light source 25 is mounted. A coupling pin 31 projectsfrom the side of the pendulum plate 22. Projecting from the other sideof the pendulum plate 22 in a position above the shaft 16 there is apivot pin 26 from which a control pendulum 27 is suspended. A photocellor light source 28 is mounted on the arm of the control pendulum 27 andthis photocell or light source 28 cooperates with the photocell or lightsource 25 of the main pendulum 24. The arm of the controller pendulum 27is interrupted by a clearance ring 29 which surrounds the shaft 16.

The free ends of the two pendulums 24 and 27 are immersed in a highlyviscous liquid contained in the lower part of the compartment 19. Theviscous liquid has a damping effect on the two pendulums for the purposeof damping out oscillations when the jib angle is changed. It should beobserved that the overload safety device is fixed to the jib by thebracket 13 and therefore follows the jib movements.

The coupling pin 31 is capable of fitting into any one of four recesses23 distributed at intervals around the face of a cam disc adjustmentplate 32 which is mounted on the shaft 16 so that it cannot rotaterelative to the shaft 16 but can slide axially along the shaft 16,against the action of a spring 33. An extension sleeve of the cam discadjustment plate 32, extending away from the pendulum plate 22, has aradially projecting rim 34 which cooperates with the rim of asurrounding housing 35. The housing 35 is mounted on a shaft 37 of awindscreen wiper motor 38 fixed to the housing 14 of the overload safetydevice. The housing 35 cannot rotate relative to the shaft 37 but canslide axially along the shaft 37 against the action of a spring 36.Fixed to the shaft 37 is a face cam 39 which cooperates with a pin 41 insuch a way that when the windscreen wiper motor 38 is set in motion thehousing 35 is moved axially along the shaft 37, towards the left as seenin FIG. 2. The rim of the housing 35 then engages frictionally with theradially projecting rim 34 of the cam disc adjustment plate 32, pullingit away from the pendulum plate 22 until the coupling pin 31 comes outof engagement with the recess 23.

Fixed to the other shaft 15 there are a number of arms 42. As shown inFIGS. 2 and 3 the arms 42 are spaced along the shaft 15, relative to thecam discs 21, in such a way that each arm 42 swings inwards between twocam discs. The arms 42 are counterbalanced by a counterweight 40. Theshaft 15 is positioned to one side of the shaft 16 so that when the,arms 42 are vertical they occupy a plane beyond the ends of the radii ofthe cam discs 21. The free ends of the arms 42 carry photocells or lightsources or the like. In the example shown the arms 42 at the extremeright in FIG. 2, furthest away from the windscreen wiper motor, has alight source 43. The arm next towards the left carries two photocells44, one of which is in line with the light source 43, the otherphotocell being displaced in position relative to the first and facingin the opposite direction. This photocell cooperates with a light source45 on the next arm towards the left, this arm having a second lightsource 46 facing towards the left. The arrangement is repeated all theway to the arm furthest to the left. This manner of arranging the lightsources and photocells reduces the amount of electrical wiring to theminimum and also keeps the photocells away from stray beams of light.The wiring can if desired be reduced still further by mounting a lampbulb 47 centrally on the shaft 15, the light from the bulb 47 beingconducted to the locations 43, 45, 46 and any remaining light sourcelocations by light conductors such as optical fibre bundles.

As shown in FIG. 1, each arm 42 can be equipped with auxiliaryphotocells 43a, 44a situated next to the photocells 43, 44, to providean early warning system.

A pulley wheel 48 is fixed on the left hand end of the shaft as seen inFIG. 2, that is to say the end nearest to the jib 12. One end of atension cable 49 is fixed to the pulley wheel 48. The cable 49 runs overa second pulley wheel 51, the other end of the cable being attached to atension rod strain gauge 52 which is fixed to the surface of the lowerweb, that is to say the compression web, of the crane jib 12. A returntension spring 54 acts on a lever 53 fixed to the shaft 15, the returnspring 54 pulling against the tension of the cable 49.

In the example illustrated, the tension rod strain gauge 52 is arrangedas follows: The tension rod 59 is installed with a tensile prestresscorresponding to the highest permissible compressive stress which can beapplied to the web of the crane jib. When the crane is in operation, ifthe jib stress reaches the highest permissible value the tension rod 59is no longer stressed in tension. As soon as the jib load increasesbeyond this point the tension rod releases itself from its twoanchorages 55. This is to ensure that if the operator disconnects theoverload safety device, for example willfully, the device neverthelesssuffers no damage.

The free end of the cable 49 is attached to a lever 56 pivoted at 57 toa frame of the gauge. In FIG. 3 the jib is assumed to be unloaded. Underthese circumstances the free end of the lever 56 is at a distance h fromits other limiting position, corresponding to the highest permissiblejib load. A return tension spring 58 holds the lever 56 in contact withan actuating finger 61 on the tension rod 59.

This example of the overload safety device operates as follows: Let itbe assumed that the jib is initially in the working position shown inFIG. 1. The pendulum 24, acting through the cam disc adjustment plate32, which cannot rotate relative to the shaft 16, rotates all the camdiscs 21 into positions corresponding to a vertical position of thependulum. This automatically compensates for any slope of the vehicle onwhich the crane jib is mounted. A load lifted by the crane and appliedto the jib swings the arms 42 inwards by means of the strain gauge andthe cable 49. If the jib load reaches the permissible limit the lightbeam of the photocell device 43, 44 is interrupted by the edge of thecam disc.

The load on the jib allows the tension rod 59 to con tract, the finger61 pivoting the lever 56 clockwise towards the tension rod. The cable 49consequently rotates the pulley wheel-48 clockwise, as seen in FIG. 1.This swings the arms 42 towards the shaft 16. If the jib load approachesthe permissible limit, the light beams of the early-waming photocells43a, 44a are first interrupte d by the edge of the cam disc 21. Thisproduces an early-warning signal. With further increase of the jib loadthe light beam of the photocell device 43, 44 is interrupted by the edgeof the cam disc, the interruption automatically prevents any cranemovements which could increase the jib load still further.

A fail-safe effect is obtained as follows. Should the connection betweenthe swinging arms 42 and the rod 59 be interrupted, for example byrupture of the cable 49, potentially dangerous crane movements areprevented by the fact that the return spring 54 immediately swings thearms 42 back into a safety light trap 62, interrupting all the relevantcircuits.

As already mentioned, each individual cam disc represents a particularjib extension, or range of extensions, whereas each of the fourquadrants of each cam disc represents a particular crane operatingcondition. When the operator changes from one operating condition toanother, for example when he swings the jib around from a longitudinalposition to a transverse position relative to the longitudinal axis ofthe crane vehicle the overload safety device is re-set to the newcondition as follows. The windscreen wiper motor 38 is started androtates the face cam 39 relative to the pin 41. This pulls the housing35 and the cam disc adjustment plate 32 away from the pendulum plate 22,disengaging the coupling pin 31 from the recess 23. The cam discadjustment plate 32 is then rotated through until the coupling pin 31engages in the nextrecess 23. During this movement the shaft 16 rotates,rotating all the cam discs 21 through 90. The movements leaves theoverload safety device re-set for the new operating condition.

The resetting of the overload safety device to a different jib extensionis a simple matter. All that is necessary is to switch over electricallyso as to connect the photocell of the relevant cam disc. Electricconnection is made between the photocell device and the safety switchsystem for preventing the crane operations. It should be observed thatonly one photocell device is connected at a given instant with thesafety switch system. Consequently even though all the swinging arms 42move simultaneously, only the particular photocell device which isconnected in the control circuit can deliver a signal to indicate thatits light beam has been interrupted by the edge of the relevant camdisc. In principle it would be possible to re-set the system for adifferent jib extension by switching on the relevant photocell lightsource. Considerable advantages are however obtained by using opticalfibre light conductors. All the light conductors are fed with light froma central light source. This arrangement has the advantage of greaterreliability and considerably less electric wiring is required.

FIGS. 4 and 5 illustrate a preferred strain-measuring device in the formof a tension rod dynamometer .lll attached to a crane jib 112, whichacts as the stressed structural part. Only a portion of the crane jib isshown in the Figures. The tension rod dynamometer 111 is attached to thecrane jib by two suspension devices which will be described in greaterdetail further below. For the reasons already mentioned the tension roddynamometer l 1 1 is installed prestressed. Moreover the two rods 113and 114 forming the dynamometer are also prestressed against each other,that is to say the inner rod 113 is prestressed against the outer rod114, the two rods being fixed to each other at one end. This is toprevent damage, particularly to the pivoted arm by collision with thecam disc axle if the prestressing is entirely cancelled by overloading,for example if the crane operator unintentionally by-passes the overloadsafety device.

A cable 115 has one end attached to the pivoted arm mentioned above, theother end of the cable being attached to the end of a pivoted lever 117which pivots on a pivot pin 116. The pivoted lever 117 is held back by atension spring 118 so that the pivoted lever rests in contact with anactuating finger 119 fixed to the inner rod 113.

The tension rod dynamometer is supported at each end by a multipointsuspension device attached to the stressed structural part 112. At eachend of the tension rod dynamometer the multipoint suspension deviceconsists of two links 121, 122 or 123, 124. One end of each link ispivoted to the structural part 112 by a pivot pin 125, 126, 127, 128.The other end of each link is pivoted to one end of the tension roddynamometer, the links being pivoted to the ends of the tension roddynamometer in pairs on common pivot pins, that is to say the two links121, 122 are pivoted to one end of the tension rod dynamometer on thecommon pivot pin 129, the other two links 123, 124 being pivoted to theother end of the tension rod dynamometer on a common pivot pin 131. Thetwo common pivot pins 129 and 131 are fixed to the two ends of thetension rod dynamometer 111.

In the present example the dynamometer is suspended as though by togglejoints by the two pairs of links 121, 122 and 123, 124 at the foursupport points 125, 126, 127, 128 of the stressed structural part 112.As a result a surprisingly effective temperature compensation isobtained in that unequal thermal expansions resulting from temperaturedifferences, which could falsify the stress measurements, arecompensated so effectively that the overload safety device functionscorrectly, responding only to the load conditions of the crane. It hasbeen found advantageous to make the distances between the supportpoints, measured longitudinally, that is to say the distance between thesupport point 125 and the support point 127, and the distance betweenthe support point 126 and the support point 128 greater than the lengthof the dynamometer 111. In other words, the distances between each twosupport points on the same side of the symmetry axis 132 are in eachcase greater than the length of the dynamometer 111. The arrangementshown in FIG. 4 is thus obtained, the two links of each pair, that is tosay the links 121 and 122 and the links 123 and 124, spreading outwardsaway from the dynamometer.

The overload safety device is particularly well suited, due to itsspecial construction, to facilitate the necessary adjustment, mentionedearlier, of the movement ratio between the movement of the pivoted lever117 and the resulting movement of the pivoted arm. This adjustment isnecessary for calibrating the overload safety device to compensate fordifferences in the thickness of the stressed structural part 112 indifferent cranes. To allow this adjustment to be made one end of thedynamometer 111 has a longitudinal slot 133 allowing the pivot pin 131to be adjusted in position by means of an adjustment screw 134. Thelengths of the links 123, 124 are also adjustable. In the presentexample each link 123, 124 consists of an externally threaded rodpivoted at one end of the pivot pin 127 or 128. Each externally threadedrod cooperates with an internally threaded part pivoted on the commonpivot pin 131. This arrangement makes it unnecessary to interfere withthe interior of the measurement or switchoff apparatus. The anglebetween the links 123 and 124 can be changed continuously, that is tosay steplessly, to allow for differences in the thickness of thestructural part 112, which of course influences the changes in thelength of the tension rod, and therefore in the angular position of thepivoted lever 117, produced by changes in the load conditions of thecrane.

The overload safety device in accordance with the invention isoutstanding in the simplicity of its construction. A furtherconsiderable advantage is that the construction is particularlyresistant to the impact shocks which occur during the operation of acrane. The device is therefore suitable for application to a widevariety of hoisting machines and related devices in which the highestpermissible working load is limited by a number of different factors.

I claim:

1. An overload safety device for a crane comprising a strain measuringdevice arranged to be mounted on the crane boom, a pivotally mounted armoperatively connected to said strain measuring device, at least onephotocell device positioned on said arm, at least one cam devicearranged to cooperate with said photocell device to signal overload onthe crane boom, wherein the improvement comprises that said strainmeasuring device is a tension rod dynamometer, and multipoint suspensionmeans for each end of said tension rod dynamometer for separatelyconnecting a common point on each end of said tension rod dynamometer tospaced attachment points on the boom.

2. A device according to claim 1, wherein each of said multipointsuspension means consists of two links each pivoted to the common pointon the end of said tension rod dynamometer, and the opposite ends ofsaid links being arranged for attachment to the spaced attachment pointson the boom.

3. A device according to claim 2, wherein said dynamometer has alongitudinal axis extending through the common points at its oppositeends, and the attachments points of said links to said boom are situatedon opposite sides of the longitudinal axis of said dynamometer and aresymmetrical with respect to the longitudinal axis.

4. A device according to claim 3, wherein the distance between each pairof said attachment points located on one side of said longitudinal axisand at the opposite ends of said dynamometer is greater than the lengthof said dynamometer.

5. A device according to claim 3, wherein the distance between each pairof attachment points located on one side of said longitudinal axis andat the opposite ends of said dynamometer is less than the length of saiddynamometer.

6. A device according to claim 2, wherein at least one end of saiddynamometer has a slot extending in the longitudinal direction thereof,a pivot pin forming the common point at one end of said dynamometer andextending through said slot and being adjustably positionable in thelongitudinal direction of the slot, and an adjustable screw operativelyconnected to said pivot pin for positioning it within the slot in thelongitudinal direction thereof.

7. A device according to claim 6, wherein said two links connected tosaid pivot pin are each extensible in length.

mometer comprises an inner rod and an outer rod coextensive with saidinner rod and said inner rod and outer rod being pre-stressed againsteach other.

1. An overload safety device for a crane comprising a strain measuringdevice arranged to be mounted on the crane boom, a pivotally mounted armoperatively connected to said strain measuring device, at least onephotocell device positioned on said arm, at least one cam devicearranged to cooperate with said photocell device to signal overload onthe crane boom, wherein the improvement comprises that said strainmeasuring device is a tension rod dynamometer, and multipoint suspensionmeans for each end of said tension rod dynamometer for separatelyconnecting a common point on each end of said tension rod dynamometer tospaced attachment points on the boom.
 2. A device according to claim 1,wherein each of said multipoint suspension means consists of two linkseach pivoted to the common point on the end of said tension roddynamometer, and the opposite ends of said links being arranged forattachment to the spaced attachment points on the boom.
 3. A deviceaccording to claim 2, wherein said dynamometer has a longitudinal axisextending through the common points at its opposite ends, and theattachments points of said links to said boom are situated on oppositesides of the longitudinal axis of said dynamometer and are symmetricalwith respect to the longitudinal axis.
 4. A device according to claim 3,wherein the distance between each pair of said attachment points locatedon one side of said longitudinal axis and at the opposite ends of saiddynamometer is greater than the length of said dynamometer.
 5. A deviceaccording to claim 3, wherein the distance between each pair ofattachment points located on one Side of said longitudinal axis and atthe opposite ends of said dynamometer is less than the length of saiddynamometer.
 6. A device according to claim 2, wherein at least one endof said dynamometer has a slot extending in the longitudinal directionthereof, a pivot pin forming the common point at one end of saiddynamometer and extending through said slot and being adjustablypositionable in the longitudinal direction of the slot, and anadjustable screw operatively connected to said pivot pin for positioningit within the slot in the longitudinal direction thereof.
 7. A deviceaccording to claim 6, wherein said two links connected to said pivot pinare each extensible in length.
 8. A device according to claim 7, whereineach of said extensible links consists of an externally threaded rod andan internally threaded cylinder in threaded engagement with said rod. 9.A device according to claim 1, wherein said dynamometer comprises aninner rod and an outer rod coextensive with said inner rod and saidinner rod and outer rod being pre-stressed against each other.